1. Field of the Invention
The present invention pertains to an industrial polyester multifilament yarn with high modulus and low shrinkage, which is useful as a fiber reinforcement material of a tire, and a method of producing the same. Compared to a prior industrial polyester multifilament yarn with high modulus and low shrinkage, the industrial polyester multifilament yarn according to the present invention is advantageous in that the polyester multifilament yarn, treated with an adhesive (RFL) and heat-treated in order to produce a tire cord, has excellent physical properties, including tenacity, elongation, toughness, shrinkage, and dimensional stability, even though a grey yarn of the above polyester multifilament yarn has poor tenacity, elongation, and toughness. Accordingly, the polyester multifilament yarn of the present invention is usefully applied to produce a treated cord with high modulus and tenacity, and low shrinkage.
2. Description of the Prior Art
Generally, a prior method of producing an industrial polyethylene terephthalate polyester fiber with high modulus and low shrinkage, includes a high-speed spinning step of spinning a yarn at a spinning rate of 2000 m/min or higher, a drawing step at a glass transition temperature or higher, that is, 80° C. or higher, typically 110° C. or higher according to a spin draw process, and a heat-setting step at 220° C. or higher.
An earliest method of producing an industrial polyethylene terephthalate fiber with high modulus and low shrinkage is disclosed in U.S. Pat. No. 4,101,525 (Herbert al Davis, et al.), in which a heat-setting process is conducted at 220 to 240° C. to produce the polyester fiber with crystallinity of 45 to 55%.
Heretofore, the crystallinity of 45 to 55% has been considered as one of standard physical properties of the industrial polyethylene terephthalate drawn yarn with high modulus and low shrinkage.
Meanwhile, U.S. Pat. No. 4,491,657 (Iso Saito, et al.) suggests a method of producing a polyethylene terephthalate drawn yarn, in which a heat-setting temperature is 220° C., and the polyester drawn yarn has terminal modulus of 0 to 15 g/d, thereby improving strength retention of a tire cord, produced using the above polyester drawn yarn.
Furthermore, U.S. Pat. No. 4,349,501 (Maxwell C. Hamlin, et al.) and U.S. Pat. No. 4,851,172 (Hudge H. Lowan, et al.) recite a drawing process using steam at 300° C. or higher, in which a draw point is fixed. As well, Japanese Pat. Laid-Open Publication No. Hei. 7-70819 (Masayasu Nagao, et al.) proposes a process of producing a polyester drawn yarn, in which a winding process is conducted at a spinning rate of 2500 to 6000 m/min, and an undrawn yarn with an intrinsic viscosity of 0.85 or more and a density of 1.365 g/cm3 or more is subjected to a multi-stage drawing process at 100° C. or lower and a heat-setting process at 225° C. However, this patent aims to produce the polyester yarn with low shrinkage similar to rayon or vinylon, and thus, it is hardly used to produce a yarn for a tire cord having high tenacity.
Particularly, the undrawn yarn with the density of 1.365 g/cm3 or more has too high crystallinity to mechanically draw. Hence, it is very difficult to produce a yarn with a high strength of 7.2 g/d or more required to produce the tire cord.
While studying on improvement of strength retention of a polyester multifilament yarn, having high strength and modulus, and low shrinkage, treated with an adhesive for rubbers (an RFL adhesive for tire cords) and then heat-treated, in a process of producing a drawn yarn using a highly oriented undrawn yarn, the present inventors found the fact that strength retention (tenacity of a heat-treated tire cord/tenacity of a grey yarn) of the tire cord, produced using a prior yarn with high tenacity, elongation, and toughness, and low terminal modulus, is relatively low. Accordingly, the present inventors reached a conclusion that in the case of a yarn for the tire cord subjected to a post-heat treating process, it is preferable to properly suppress crystallization of the yarn by heat so as to increase tenacity of a final product, produced using the above yarn, and to improve strength retention of the final product after the yarn is subjected to the post-heat treating process.